IA Receptor Research Articles

Smooth Muscle Protein 22α–Mediated Patchy Deletion of Bmpr1a Impairs Cardiac Contractility but Protects Against Pulmonary Vascular Remodeling

Vascular expression of bone morphogenetic type IA receptor (Bmpr1a) is reduced in lungs of patients with pulmonary arterial hypertension, but the significance of this observation is poorly understood. To elucidate the role of Bmpr1a in the vascular pathology of pulmonary arterial hypertension and associated right ventricular (RV) dysfunction, we deleted Bmpr1a in vascular smooth muscle cells and in cardiac myocytes in mice using the SM22alpha;TRE-Cre/LoxP;R26R system. The LacZ distribution reflected patchy deletion of Bmpr1a in the lung vessels, aorta, and heart of SM22alpha;TRE-Cre;R26R;Bmpr1a(flox/+) and flox/flox mutants. This reduction in BMPR-IA expression was confirmed by Western immunoblot and immunohistochemistry in the flox/flox group. This did not affect pulmonary vasoreactivity to acute hypoxia (10% O2) or the increase in RV systolic pressure and RV hypertrophy following 3 weeks in chronic hypoxia. However, both SM22alpha;TRE-Cre;R26R;Bmpr1a(flox/+) and flox/flox mutant mice had fewer muscularized distal pulmonary arteries and attenuated loss of peripheral pulmonary arteries compared with age-matched control littermates in hypoxia. When Bmpr1a expression was reduced by short interference RNA in cultured pulmonary arterial smooth muscle cells, serum-induced proliferation was attenuated explaining decreased hypoxia-mediated muscularization of distal vessels. When Bmpr1a was reduced in cultured microvascular pericytes by short interference RNA, resistance to apoptosis was observed and this could account for protection against hypoxia-mediated vessel loss. The similar elevation in RV systolic pressure and RV hypertrophy, despite the attenuated remodeling with chronic hypoxia in the flox/flox mutants versus controls, was not a function of elevated left ventricular end diastolic pressure but was associated with increased periadventitial deposition of elastin and collagen, potentially influencing vascular stiffness.

Bone morphogenetic protein (BMP)-7 but not BMP-2 and BMP-4 improves maintenance of primitive peripheral blood-derived hematopoietic progenitor cells (HPC) cultured in serum-free medium supplemented with early acting cytokines

BMPs regulate the developmental program of hematopoiesis. We demonstrate an increased expression of the BMP receptors Ia and II on cultured CD34 + cells and examine the impact of BMP-2, -4 and -7 on postnatal HPC cultured with stem cell factor, flt3-ligand and interleukin-3 (SF3). The addition of BMP-2 at 5, 25 and 50 ng/m to serum-free medium with SF3 yielded a 1.4- to 1.2-fold increase of CD34 + cells after seven days, but no effect on CFC or LTC-IC was observed. BMP-4 at 25 ng/ml induced a 2.9-fold expansion of colony-forming cells (CFC) within 1 week followed by a decrease to pre-culture values on day 14. The number of long-term culture initiating cells (LTC-IC) decreased by the factor 40 from day 0 to day 14. BMP-7 at 5–50 ng/ml had not effect on the expansion of CD34 + cells and CFC, but improved at 5 ng/ml the survival of LTC-IC significantly as compared to SF3 alone. In summary, BMP-2, -4 and -7 have no effect on the proliferation of CD34 + cells and CFC cultured with serum-free medium and SF3. However, BMP-7 but not BMP-2 and BMP-4 prevents the loss of primitive hematopoietic progenitor cells cultured in SFM plus SF3.

Viral Tropism and the Pathogenesis of Influenza in the Mammalian Host

Influenza viruses, single-stranded negative-sense RNA viruses within the family Orthomyxoviridae, remain an important cause of human morbidity and mortality worldwide. Despite effective vaccines, more than 200,000 individuals in the United States are hospitalized each year for influenza, and approximately 35,000 succumb to disease.1 Seasonal epidemics arise through antigenic variation and recombination events from existing and currently circulating human strains, which initially target the upper respiratory tract with extension to a bronchointerstitial pneumonia in a subset of affected individuals. Disease is often more severe in older and debilitated patients, producing a characteristic distribution in age-related mortality within the affected population.2 In contrast to seasonal epidemics, pandemics arise when antigenically novel viruses emerge and are readily transmissible within the naive human population. Such events are rare and occur after zoonotic transmission of viruses through recombination events between established human and avian strains or possibly through direct adaptation of avian strains for efficient human to human transmission.3 Concerns over an avian influenza pandemic have risen since 1997 with the recognition of H5N1 as a cause of fulminant disease in humans.4,5 Pathologically fatal H5N1 causes diffuse alveolar damage and progression to multiple organ dysfunction with a case fatality rate estimated at more than 50%. Although H5N1 disease is severe, transmission rates have been low, and viral adaptation to efficient human to human transmission has not occurred. Understanding viral and host barriers that prevent transmission may be critical in establishing rational control measures as well as predicting and stratifying risk for individual strains of influenza. In this issue of The American Journal of Pathology, van Riel et al6 examine the viral binding distribution in pulmonary tissue of a number of avian and human influenza strains to elucidate the role of tropism in transmission and pathogenesis.

BMP7: A New Bone Metastases Prevention?

Prostate cancer is the most frequently diagnosed cancer in men and a leading cause of cancer death. More than 218,000 cases will occur in 2007, and prostate cancer will lead to more than 27,000 deaths.1 Although the 5-year survival rate is excellent for localized stages (100%), the relative survival rapidly decreases to 33% when prostate cancer metastasizes.1 Cancer cells from prostate primary tumors commonly colonize bones, and postmortem analyses show that 65 to 75% of patients with advanced disease have bone metastases.2 Scientific knowledge of bone metastasis pathophysiology has increased in recent years, but effective therapy for this devastating complication of cancer remains suboptimal. In this issue of The American Journal of Pathology, Buijs and colleagues3 shed more light into pathophysiology of bone metastases. More importantly, they apply this knowledge to develop novel therapy for bone metastases using bone morphogenetic protein 7 (BMP7).

Expression of bone morphogenetic proteins 4 and 7 and their receptors IA, IB, and II in human ovaries from fetuses and adults

To investigate the immunocytochemical and mRNA expression of bone morphogenetic proteins 4 (BMP-4) and 7 (BMP-7) and their three receptors (BMPR-IA, BMPR-IB, and BMPR-II) in ovaries from human adults and fetuses. Immunocytochemical and in situ hybridization study. Major tertiary care and referral academic centers. Sixteen adolescents/adults aged 13-38 years and 31 women undergoing second and third trimester pregnancy terminations. None. Immunocytochemistry and in situ hybridization on paraffin sections of human ovaries from fetuses and adults. The expression of the proteins for BMP-4 and BMP-7 and their receptors was detected in oogonia/oocytes and stroma cells from both sources (fetuses and women/adolescents). BMP-7 and the three receptors were identified in all of these granulosa cells, and BMP-4 was detected only in the granulosa cells of women/adolescents. Transcripts for all five ligands were identified in stroma cells of all samples and in fetal oogonia/oocytes. BMPR-IA and BMPR-IB mRNA expression was also identified in oocytes from women/adolescents. BMP-7 and BMPR-IA mRNA staining was detected in fetal granulosa cells. This is the first report of the expression of BMP-4 and BMP-7 and their receptors in human ovaries from fetuses as well as adults. However, to elucidate whether indeed BMP-4 or BMP-7 are involved in the initiation of human primordial follicular growth, they should be added to the culture medium.

BMP2, BMP4, and their receptors are expressed in the differentiating muscle tissues of mouse embryonic tongue

To investigate the role of bone morphogenetic proteins (BMPs) in the differentiation process of skeletal muscle, we analyzed the in vivo expression of BMP2 and BMP4, of BMP receptors (BMPR) IA, IB, and II, and of activin receptors (ActR) IA, II, and IIB in mouse tongue muscle between embryonic day 11 (E11) and E17. The mRNA expression levels for BMP2 were 5-fold to 11-fold greater than those for BMP4 between E13 and E17 (P < 0.05-0.01). Expression of the BMP2, BMPRIB, ActRIA, ActRII, and ActRIIB proteins was first observed at E13. Expression of BMP2 and BMPRIB was detected in the whole area of the differentiating muscle tissues identified by immunostaining for fast myosin heavy chain (fMHC), but that of ActRIA, ActRII, and ActRIIB was detected only in the peripheral area of the differentiating muscle tissues. In the E15 tongue, all of the BMPs, BMPRs, and ActRs studied herein were expressed in the whole area of the differentiating muscle tissues identified by immunostaining for fMHC. These results suggest that BMPs play a role in the differentiation of tongue muscle tissues at E15 but have little or no effect at E13.

Octaarginine-modified multifunctional envelope-type nanoparticles for gene delivery

This study describes a multifunctional envelope-type nano device (MEND) that mimics an envelope-type virus based on a novel packaging strategy. MEND particles contain a DNA core packaged into a lipid envelope modified with an octaarginine peptide. The peptide mediates internalization via macropinocytosis, which avoids lysosomal degradation. MEND-mediated transfection of a luciferase expression plasmid achieved comparable efficiency to adenovirus-mediated transfection, with lower associated cytotoxicity. Furthermore, topical application of MEND particles containing constitutively active bone morphogenetic protein (BMP) type IA receptor (caBmpr1a) gene had a significant impact on hair growth in vivo. These data demonstrate that MEND is a promising non-viral gene delivery system that may provide superior results to existing non-viral gene delivery technologies.

Bone Morphogenetic Protein Signaling Inhibits Hair Follicle Anagen Induction by Restricting Epithelial Stem/Progenitor Cell Activation and Expansion

Epithelial stem cells (EP-SCs) located in the bulge region of a hair follicle (HF) have the potential to give rise to hair follicle stem/progenitor cells that migrate down to regenerate HFs. Bone morphogenetic protein (BMP) signaling has been shown to regulate the HF cycle by inhibiting anagen induction. Here we show that active BMP signaling functions to prevent EP-SC activation and expansion. Dynamic expression of Noggin, a BMP antagonist, releases EP-SCs from BMP-mediated restriction, leading to EP-SC activation and initiation of the anagen phase. Experimentally induced conditional inactivation of the BMP type IA receptor (Bmpr1a) in EP-SCs leads to overproduction of HF stem/progenitor cells and the eventual formation of matricomas. This genetic manipulation of the BMP signaling pathway also reveals unexpected activation of beta-catenin, a major mediator of Wnt signaling. We propose that BMP activity controls the HF cycle by antagonizing Wnt/beta-catenin activity. This is at least partially achieved by BMP-mediated enhancement of transforming growth factor-beta-regulated epithelial cell-specific phosphatase (PTEN) function. Subsequently, PTEN, through phosphatidyl inositol 3-kinase-Akt, inhibits the activity of beta-catenin, the convergence point of the BMP and Wnt signaling pathways.

Platelet glycoprotein polymorphisms: Risk, in vivo expression and severity of atherothrombotic stroke in Chinese

Background Polymorphisms in platelet glycoprotein (GP) receptors Ia, Ib and IIIa may be heritable risk factors for platelet-dependent thrombosis leading to death. The precipitation of stroke by occlusive thrombi has led to the investigation of the platelet surface GP receptors, that are involved in critical steps in the activation of platelets. Three polymorphisms in the GP Iba gene and one in each of GPIIIa, GP Ia were selected based on the evidence of functional effects on structure or expression as candidates for risk. We also determined whether these polymorphisms were associated with in vivo expression levels of platelet GP receptors and the severity of the neurological deficit. Methods A Chinese hospital-based case-control study was conducted with 119 cases of atherothrombotic stroke and 166 age and sex matched controls. Genotyping was performed on lymphocyte DNA by standard methods and platelet GP expression levels were measured by flow-cytometry. Results Allele and genotype frequencies of the GP receptor polymorphisms differ considerably between ethnic populations. We found the D allele of the GP Iba VNTR polymorphism was significantly associated with atherothrombotic stroke in our Chinese cohort, however we did not find a relationship among these polymorphisms, the expression levels of GP receptors and severity of the neurological deficit. Conclusions In our Chinese cohort the D allele of the GP Iba VNTR polymorphism is associated with atherothrombotic stroke. The number of VNTR repeats alters the length of amino acid sequence, which might affect the structure and function of this receptor.

BMP signaling in the epiblast is required for proper recruitment of the prospective paraxial mesoderm and development of the somites

Bmpr1a encodes the BMP type IA receptor for bone morphogenetic proteins (BMPs), including 2 and 4. Here, we use mosaic inactivation of Bmpr1a in the epiblast of the mouse embryo (Bmpr-MORE embryos) to assess functions of this gene in mesoderm development. Unlike Bmpr1a-null embryos, which fail to gastrulate, Bmpr-MORE embryos initiate gastrulation, but the recruitment of prospective paraxial mesoderm cells to the primitive streak is delayed. This delay causes a more proximal distribution of cells with paraxial mesoderm character within the primitive streak, resulting in a lateral expansion of somitic mesoderm to form multiple columns. Inhibition of FGF signaling restores the normal timing of recruitment of prospective paraxial mesoderm and partially rescues the development of somites. This suggests that BMP and FGF signaling function antagonistically during paraxial mesoderm development.

Heparin Potentiates the in Vivo Ectopic Bone Formation Induced by Bone Morphogenetic Protein-2

Although bone morphogenetic proteins (BMPs) are clinically useful for bone regeneration, large amounts are required to induce new bone formation in monkeys and humans. We found recently that heparin stimulates BMP activity in vitro (Takada, T., Katagiri, T., Ifuku, M., Morimura, N., Kobayashi, M., Hasegawa, K., Ogamo, A., and Kamijo, R. (2003) J. Biol. Chem. 278, 43229-43235). In the present study, we examined whether heparin enhances bone formation induced by BMPs in vivo and attempted to determine the molecular mechanism by which heparin stimulates BMP activity using C2C12 myoblasts. Heparin enhanced BMP-2-induced gene expression and Smad1/5/8 phosphorylation at 24 h and thereafter, although not within 12 h. Heparitinase treatment did not affect the response of cells to BMP-2. In the presence of heparin, degradation of BMP-2 was blocked, and the half-life of BMP-2 in the culture medium was prolonged by nearly 20-fold. Although noggin mRNA was induced by BMP-2 within 1 h regardless of the presence of heparin, noggin failed to inhibit BMP-2 activity in the presence of heparin. Furthermore, simultaneous administration of BMP-2 and heparin in vivo dose-dependently induced larger amounts of mineralized bone tissue compared with BMP-2 alone. These findings clearly indicate that heparin enhances BMP-induced osteoblast differentiation not only in vitro but also in vivo. This study indicates that heparin enhances BMP-induced osteoblast differentiation in vitro and in vivo by protecting BMPs from degradation and inhibition by BMP antagonists.

Expression of the TGF-β Family of Ligands Is Developmentally Regulated in Skeletal Muscle of Neonatal Rats

To dissect the possible role of the transforming growth factor-beta (TGF-beta) family in the regulation of skeletal muscle growth during the early postnatal period, the protein abundances of the TGF-beta family and their correlation with protein synthesis were determined in skeletal muscle of neonatal rats. To obtain direct evidence of the role of these growth factors in the regulation of protein synthesis, the TGF-beta inhibitor, follistatin, was infused into 10-d-old rats for 11 d and protein synthesis and phosphorylation of S6 kinase 1 (S6K1) and ribosomal protein (rpS6) were measured. TGF-beta2 abundance and protein synthesis in muscle decreased with development and were positively correlated. The abundances of bone morphogenetic protein 2 (BMP-2), BMP-7, and myostatin increased with development and were negatively correlated with protein synthesis. The abundances of BMP-2 and BMP-7 were positively correlated with BMP receptor IA (BMP-RIA) abundance. Activin A abundance was positively correlated with follistatin abundance and activin receptor IIB (Act-RIIB) abundance. Infusion of follistatin increased muscle protein synthesis and S6K1 and rpS6 phosphorylation. The results provide indirect and direct evidence of TGF-beta family involvement in the regulation of muscle protein synthesis during the neonatal period.

Mutual Regulation of Follicle-Stimulating Hormone Signaling and Bone Morphogenetic Protein System in Human Granulosa Cells1

Bone morphogenetic proteins (BMPs) play critical roles in folliculogenesis by modulating the actions of follicle-stimulating hormone (FSH) in the ovary. However, the effects of FSH on the BMP system remain unknown. Here, we have investigated the effects of FSH on BMP signaling using the human granulosa-like tumor cell line KGN. KGN cells express BMP type I and type II receptors and the BMP signaling molecules SMADs. FSH administration upregulated BMP type IA (BMPR1A) and IB (BMPR1B) receptors, activin type II receptor (ACVR2), and BMP type II receptor (BMPR2). FSH also augmented SMAD1 and SMAD5 expression, and conversely, FSH suppressed the expression of the inhibitory SMADs, SMAD6 and SMAD7. Bioassays revealed that FSH enhances BMP-induced SMAD1/5/8 phosphorylation and cellular DNA synthesis induced by BMP6 and BMP7. Since overexpression of BMPR1A and BMPR1B, but not SMADs, significantly enhanced the BMP responses, these type I receptors were revealed to be limiting factors for BMP signaling in KGN cells. BMPs significantly suppressed progesterone synthesis induced by forskolin and dibutyryl-cAMP (BtcAMP) but had no effect on estradiol induced by the same factors. KGN cAMP levels induced by forskolin were not altered by BMPs, suggesting that BMPs regulate steroidogenesis at a level downstream of cAMP synthesis in KGN cells. In this regard, BMPs specifically reduced the STAR transcription, whereas the levels of CYP11A, HSD3B2, and CYP19 stimulated by forskolin as well as BtcAMP were not altered. Collectively, the two major factors, FSH-cAMP pathway and BMP system, are reciprocally and functionally linked. Given that BMPs downregulate FSH receptors in KGN cells, this interaction may contribute to fine-tuning of the mutual sensitivity toward BMP ligands and FSH.

Expression of Growth Differentiation Factor 9 in the Oocytes Is Essential for the Development of Primordial Follicles in the Hamster Ovary

Postnatal growth differentiation factor 9 (GDF-9) expression in the hamster oocytes precedes the formation of primordial follicles. We examined the functional significance of GDF-9 in primordial folliculogenesis in the hamster ovary using RNA interference knockdown of GDF-9 mRNA and protein expression. Fifteen-day-old fetal ovaries were cultured for 9 d with or without 1 ng FSH, 1 microl Metafectane, 100 nM control nontargeting small interfering RNA (siRNA), GDF-9 siRNA, or GDF-9 siRNA + FSH, and the development of primordial follicles examined. The efficiency of siRNA transfecting ovarian cells in the organ culture was tested by culturing ovaries with siGlo, a nontargeting control siRNA labeled with Cy3. More than 90% of cells in the ovary were siGlo positive, and neither the Metafectane nor the siRNA-induced cellular apoptosis. Control siRNA did not affect the basal levels of GDF-9 mRNA, but GDF-9 siRNA slightly but significantly reduced the level. FSH markedly up-regulated the levels of GDF-9 mRNA and protein, and the effect was completely suppressed by GDF-9 siRNA. However, GDF-9 siRNA did not affect the levels of bone morphogenetic protein receptor IA or beta-actin mRNA. GDF-9 siRNA alone also reduced GDF-9 protein expression. Concurrent with GDF-9 expression, FSH significantly augmented primordial follicle formation, but the effect was abolished by GDF-9 siRNA. These results suggest that endogenous GDF-9 plays an important role in somatic cell differentiation and the formation of primordial follicles. Furthermore, FSH, by virtue of regulating GDF-9 expression, modulates oocyte regulation of primordial follicles formation.

Bone morphogenetic proteins: from structure to clinical use

Bone morphogenetic proteins (BMPs) are multi-functional growth factors belonging to the transforming growth factor ss superfamily. Family members are expressed during limb development, endochondral ossification, early fracture, and cartilage repair. The activity of BMPs was first identified in the 1960s but the proteins responsible for bone induction were unknown until the purification and cloning of human BMPs in the 1980s. To date, about 15 BMP family members have been identified and characterized. The signal triggered by BMPs is transduced through serine/threonine kinase receptors, type I and II subtypes. Three type I receptors have been shown to bind BMP ligands, namely: type IA and IB BMP receptors and type IA activin receptors. BMPs seem to be involved in the regulation of cell proliferation, survival, differentiation and apoptosis, but their hallmark is their ability to induce bone, cartilage, ligament, and tendon formation at both heterotopic and orthotopic sites. This suggests that, in the future, they may play a major role in the treatment of bone diseases. Several animal studies have illustrated the potential of BMPs to enhance spinal fusion, repair critical-size defects, accelerate union, and heal articular cartilage lesions. Difficulties in producing and purifying BMPs from bone tissue have prompted the attempts made by several laboratories, including ours, to express these proteins in the recombinant form in heterologous systems. This review focuses on BMP structure, molecular mechanisms of action and significance and potential applications in medical, dental and veterinary practice for the treatment of cartilage and bone-related diseases.

Osteointegration of hydroxyapatite-titanium implants coated with nonglycosylated recombinant human bone morphogenetic protein-2 (BMP-2) in aged sheep

Osteointegration of metal implants into aged organisms can be severely compromised due to reduced healing capacity of bone, lack of precursor cells for new bone formation, or osteoporosis. Here, we report on successful implant healing in a novel model of aged sheep in the presence of nonglycosylated bone morphogenetic protein 2 (BMP-2). Ewes of 8 to 12 years with significant radiologic and histologic signs of osteoporosis and adipocytic bone marrow received a cylindrical hydroxyapatite-titanium implant of 12 × 10 mm. BMP-2 has been produced as a bacterial recombinant fusion protein with maltose-binding protein and in vitro generation of mature BMP-2 by renaturation and proteolytic cleavage. A BMP-2 inhibition ELISA was developed to measure the in vitro release kinetics of bioactive human BMP-2 from immersed solid implant materials by using Escherichia coli expressed and biotinylated recombinant human BMP-2 receptor IA extracellular domain (ALK-3 ECD). The implants were placed laterally below both tibial plateaus, with the left leg implant carrying 380 μg BMP-2. Both implant types became integrated within the following 20 weeks. The control implant only integrated at the cortical bone, and little new bone formation was found within the pre-existing trabecular bone or the marrow cavity. Marrow fat tissue was partially replaced by unspecific connective tissue. In contrast, BMP-2-coated implants initiated significant new bone formation, initially in trabecular arrangements to be replaced by cortical-like bone after 20 weeks. The new bone was oriented towards the cylinder. Highly viable bone marrow appeared and filled the lacunar structures of the new bone. In mechanical tests, the BMP-2-coated implants displayed in average 50% higher stability. This animal model provided first evidence that application of nonglycosylated BMP-2 coated on solid implants may foster bone healing and regeneration even in aged-compromised individuals.

Alk3/Bmpr1a Receptor Is Required for Development of the Atrioventricular Canal Into Valves and Annulus Fibrosus

Endocardial cushions are precursors of mature atrioventricular (AV) valves. Their formation is induced by signaling molecules originating from the AV myocardium, including bone morphogenetic proteins (BMPs). Here, we hypothesized that BMP signaling plays an important role in the AV myocardium during the maturation of AV valves from the cushions. To test our hypothesis, we used a unique Cre/lox system to target the deletion of a floxed Alk3 allele, the type IA receptor for BMPs, to cardiac myocytes of the AV canal (AVC). Lineage analysis indicated that cardiac myocytes of the AVC contributed to the tricuspid mural and posterior leaflets, the mitral septal leaflet, and the atrial border of the annulus fibrosus. When Alk3 was deleted in these cells, defects were seen in the same leaflets, ie, the tricuspid mural leaflet and mitral septal leaflet were longer, the tricuspid posterior leaflet was displaced and adherent to the ventricular wall, and the annulus fibrosus was disrupted resulting in ventricular preexcitation. The defects seen in mice with AVC-targeted deletion of Alk3 provide strong support for a role of Alk3 in human congenital heart diseases, such as Ebstein's anomaly. In conclusion, our mouse model demonstrated critical roles for Alk3 signaling in the AV myocardium during the development of AV valves and the annulus fibrosus.

Persistent Cardiac Aldosterone Synthesis in Angiotensin II Type 1A Receptor–Knockout Mice After Myocardial Infarction

The renin-angiotensin-aldosterone system is implicated in the pathogenesis of heart failure. Pharmacological blockade of angiotensin II (Ang II)-dependent signaling is clinically effective in reducing cardiovascular events after myocardial infarction (MI) but still fails to completely prevent remodeling. The molecular basis underlying this Ang II-independent remodeling is unclear. Acute MI was induced by coronary ligation in wild-type (WT) and angiotensin II type IA receptor-knockout (AT1A-KO) mice. Left ventricular (LV) geometry, hemodynamics, and cardiac gene expression were evaluated on day 28. Severe LV remodeling and resultant cardiac dysfunction were observed in WT mice, whereas less marked, but still significant, LV remodeling and cardiac dysfunction were induced in AT1A-KO mice. Gene expression levels of aldosterone synthase and the cardiac aldosterone content were both elevated in the MI hearts, even in AT1A-KO mice. In AT1A-KO mice treated with spironolactone (20 mg/kg per day), LV remodeling, cardiac dysfunction, and cardiac gene expression of collagens and natriuretic peptides were almost normalized. Our results indicate that genetic blockade of AT1A signaling fails to arrest aldosterone production in cardiac tissues and that cardiac aldosterone plays a critical role in post-MI LV remodeling. The results suggest that spironolactone could be potentially effective in patients with MI, when used in combination with renin-angiotensin system blockade, by blocking the actions of aldosterone produced by Ang II-independent mechanisms.

A Single Residue of GDF-5 Defines Binding Specificity to BMP Receptor IB

Growth and differentiation factor 5 (GDF-5), a member of the TGF-beta superfamily, is involved in many developmental processes, like chondrogenesis and joint formation. Mutations in GDF-5 lead to diseases, e.g. chondrodysplasias like Hunter-Thompson, Grebe and DuPan syndromes and brachydactyly. Similar to other TGF-beta superfamily members, GDF-5 transmits signals through binding to two different types of membrane-bound serine-/threonine-kinase receptors termed type I and type II. In contrast to the large number of ligands, only seven type I and five type II receptors have been identified to date, implicating a limited promiscuity in ligand-receptor interaction. However, in contrast to other members of the TGF-beta superfamily, GDF-5 shows a pronounced specificity in type I receptor interaction in cross-link experiments binding only to BMP receptor IB (BMPR-IB). In mice, deletion of either GDF-5 or BMPR-IB results in a similar phenotype, indicating that GDF-5 signaling is highly dependent on BMPR-IB. Here, we demonstrate by biosensor analysis that GDF-5 also binds to BMP receptor IA (BMPR-IA) but with approximately 12-fold lower affinity. Structural and mutational analyses revealed a single residue of GDF-5, Arg57 located in the pre-helix loop, being solely responsible for the high binding specificity to BMPR-IB. In contrast to wild-type GDF-5, variant GDF-5R57A interacts with BMPR-IA and BMPR-IB with a comparable high binding affinity. These results provide important insights into how receptor-binding specificity is generated at the molecular level and might be useful for the generation of receptor subtype specific activators or inhibitors.

Coulombic and Hydrophobic Interactions in the First Intracellular Loop Are Vital for Bradykinin B2 Receptor Ligand Binding and Consequent Signal Transduction

The role of the first intracellular loop (IC1) in the function of the rat bradykinin B2 receptor (BKB2R) was probed. On the basis of the bovine rhodopsin X-ray structure, the BKB2R IC1 consists of six residues: (60)HKTNCT. Exchange of this sequence with the bradykinin B1 receptor IC1 (PRRQLN) resulted in a chimera which bound bradykinin and signaled as wild-type (WT) BKB2R. In contrast, a chimera containing the IC1 of rat angiotensin II type Ia receptor (AT1aR) (YMKLKT) did not bind BK nor signal in response to BK at a concentration as high as 5 microM. ELISA illustrated that this receptor was still processed and inserted into the plasma membrane. Employing portions of the IC1, we observed that (60)HKT of BKB2R could be exchanged as a group with either the BKB1R (PRR) or AT1aR (YMK) with no change in receptor binding or signaling activities. When only the YM of AT1aR replaced the HK of BKB2R, leaving the N-terminal portion of IC1 without a positively charged residue, binding and signaling were reduced by more than 70%. When only N63 was replaced with the corresponding leucine of AT1aR, binding and signaling were ablated. In fact, replacement of the entire IC1 with the AT1aR except for N63 resulted in binding and signaling as WT BKB2R. However, N63 could be replaced by glutamine (in BKB1R) or aspartate and continued to function as WT BKB2R. NMR data indicated that the BKB2R IC1 extends beyond the bovine rhodopsin prototype to include HKTNCTVAEI. When E68 was exchanged with a serine (in AT1aR), ligand binding decreased by 60% and PI turnover decreased by 69%. Molecular modeling points to a strict requirement for a hydrophilic residue at position 63 (N) at the middle of the IC1 and a Coulombic charge interaction between the positive charges (H60 and K61) at the N-terminus and a negative charge (E68) at the C-terminus of the IC1.